In Tougen Anki, the bloodlines of the Oni and Momotarou have remained at war for thousands of years. The Oni are demon-like humans whose own ferocity once drove them into seclusion, but that fragile peace was broken by an attack from Momotarou, setting off a conflict that eventually hardened into opposing powers known as the Oni Agency and the Momotarou Agency. In the present day, temperamental gun enthusiast Shiki Ichinose seems to be living a carefree life, until his expulsion from school and a sudden attack drag him into a violent struggle he never expected. As his adoptive father Tsuyoshi pulls him to safety, Shiki learns the truth of his bloodline: he is descended from the Oni, while both Tsuyoshi and their pursuer belong to the Momotarou lineage.
The revelation erupts into a fierce battle in which Tsuyoshi sacrifices himself to protect Shiki, triggering the power hidden in Shiki's Oni blood. The awakening grants him immense strength at the cost of sending him berserk, and although the enemy escapes, the incident draws the attention of Naito Mudano, an Oni teacher at Rasetsu Academy. Under Mudano's guidance, Shiki enters the Oni world to learn control, seek vengeance for his father, and survive a hereditary war that also links him to figures such as Jin Kougasaki.
